Along with the popularization of personal computers, it has become easy to input an image by an image input device such as a digital camera, color scanner, or the like, to display and confirm that image using an image display device such as a CRT, LCD, or the like, to apply an edit process, modification process, retouching process, and the like to the image in accordance with the intended purpose, and to output the image using an image output device such as a color printer or the like. Also, an image sensed by a digital camera is often directly output from a color printer without being displayed on a color monitor. Of course, when a CG image created on a computer is output by a color printer, no image input device is required.
Image input/output devices such as a digital camera, color scanner, color monitor, color printer, and the like have different color reproduction characteristics and ranges. In order to match color reproduction between image input/output devices, a standard color space is used as a color space to exchange color signals between devices to execute a color process that places an importance on gradation (tincture), vividness, color difference, or the like in correspondence with the input/output purpose or application of an image.
As a standard color space used to exchange color signals between devices, sRGB as a monitor color space is known. The gamut of the sRGB color space is narrower than that of an ink-jet printer. As standard color spaces having broader gamuts, sYCC, bg-sRGB, scRGB, and the like are known. However, these color spaces have broad gamuts beyond the human visible range (visual limit range), but do not define handling of data outside the visual limit range.
The human visible range (visual limit range) indicates a region in the xy chromaticity diagram, which is surrounded in a spectrum locus and a purple boundary, and indicates a color reproduction range which is a visible range of the human.
As for handling of data outside a gamut, Japanese Patent Laid-Open No. 4-186969 discloses an example wherein when a color outside the gamut of an image output device is input, color conversion that attaches an importance on color differences and converts the input color into a color which is closest to the input color (i.e., a color having a minimum color difference) within the gamut of the image output device is executed. However, the technique disclosed in this reference not only does not check whether the input color falls inside or outside the visual limit range, but also does not consider the tincture of original input data since that technique simply converts a color outside the gamut of the image output device into a color having a minimum color difference.
As for handling of extended-gamut digital image data, Japanese Patent Laid-Open No. 2002-27275 obtains a digital image of a preservation space by adjusting the color value of an extended-gamut digital image to fall within a limited gamut, and expressing the limited-gamut digital image within a preservation color space. Then, this technique determines a residual image that indicates the difference between the extended-gamut digital image and limited-gamut digital image to associate them with the digital image of the preservation space. However, in the contents of Japanese Patent Laid-Open No. 2002-27275, a preservation image and residual image are used to preserve a target image, and how to handle color space in an image process between color input/output devices that handle a color image with a broad gamut is not specified.